(1) Field of the Invention
The present invention relates to a thermal head for thermal recording and a method for the fabrication thereof. More particularly, the invention relates to an improved heat-generating resistor for a thermal head, which has a high specific resistance and hence, generates a large quantity of heat per unit electric current and which is excellent in the uniformity of generation of heat and also in the bondability to an electric conductor. Furthermore, the present invention relates to a method for forming a heat-generating resistor on a substrate by sputtering or electron beam vacuum deposition.
(2) Description of the Prior Art
A conventional thermal head for thermal recording has a laminate structure obtained, for example, by laminating a heat-generating resistor composed of tantalum nitride (Ta.sub.2 N), an electric conductor composed of a metal such as aluminum (Al), gold (Au) or copper (Cu) and a protecting film composed of tantalum pentoxide (Ta.sub.2 O.sub.5) or the like in order on a substrate composed of an electrically insulating material such as alumina. By applying a certain electric power to the heat-generating resistor through the electric conductor, Joule heat generation is caused in the heat-generating resistor and the function of the thermal head is exerted.
In this conventional thermal head, however, since the specific resistance of tantalum nitride constituting the heat-generating resistor is relatively small, for example 260 .mu..OMEGA.-cm, and in order to produce a desired temperature in the heat-generating resistor in a moment (within 0.5 to 2 milliseconds), a large electric current should be caused to flow in the heat-generating resistor, and a power source for supplying a large electric current is necessary. Furthermore, in case of a thermal head having a plurality of electric conductors connected to a common conductor, because of the power loss by resistances of the electric conductors and common conductor, there is brought about a difference of the quantity of the electric current flowing in the heat-generating resistor between the case where generation of heat is simultaneously caused in many heat-generating resistors and the case where generation of heat is effected in a small number of heat-generating resistors, and this difference of the quantity of the electric current results in deviation of the printing density.
Moreover, in this conventional thermal head, since tantalum nitride constituting the heat-generating resistor has poor affinity with aluminum, gold or copper constituting the electric conductor, the bonding strength between the heat-generating resistor and the electric conductor is extremely low and if an external force is applied at the time of the operation of the thermal head, for example, the electric conductor is readily peeled from the heat-generating resistor and it becomes impossible to apply a predetermined voltage. Accordingly, it is impossible to elevate the temperature of the heat-generating resistor to a desired level by Joule heat generation, with the result that the thermal head fails to duly function or printing cannot be satisfactorily performed.